[I]t may not be amiss to observe from these definitions of natural and unnatural, that nothing can be more unphilosophical than those systems, which assert, that virtue is the same with what is natural, and vice with what is unnatural. For in the first sense of the word, Nature, as opposed to miracles, both vice and virtue are equally natural; and in the second sense, as oppos’d to what is unusual, perhaps virtue will be found to be the most unnatural. At least it must be own’d, that heroic virtue, being as unusual, is as little natural as the most brutal barbarity. As to the third sense of the word, ’tis certain, that both vice and virtue are equally artificial, and out of nature. For however it may be disputed, whether the notion of a merit or demerit in certain actions be natural or artificial, ’tis evident, that the actions themselves are artificial, and are perform’d with a certain design and intention; otherwise they cou’d never be rank’d under any of these denominations. ‘Tis impossible, therefore, that the character of natural and unnatural can ever, in any sense, mark the boundaries of vice and virtue.

David Hume, A Treatise of Human Nature, Edinburgh, 1739, bk. 3, pt. 1, sect. 2